The present invention relates to a projection lens system suitable for an optical system in a projection type television in which a projection image superior in contrast is obtained.
A projection type television is constituted in such a manner that images on fluorescent faces of three monochromatic projection type cathode-ray tubes which emit, for example, red, green and blue, respectively, are enlarged by projection lenses provided in front of the cathode-ray tubes, then projected onto a screen and combined together on the screen to obtain a color image. FIGS. 6 and 7 each show a general arrangement example of optical parts of a projection type television. In these figures, the reference numerals 10, 11, 12 and 13 denote a projection type cathode-ray tube, a projection lens, a housing and a screen, respectively, and numerals 18 and 19 represent turn-back or reflecting mirrors. In the case of using a lens system with a long projection distance, two turnback or reflecting mirrors 18 and 19 disposed so as to reduce the entire size, as shown in FIG. 7, while in the case of a lens system with a short projection distance, it is sufficient to use only one mirror 18, as shown in FIGS. 6A and 6B. Since a mirror may cause deterioration of focus and contrast, development of projection lenses having a short projection distance, a reduced aberration and a good focusing characteristic has been conducted as described, for example, in Japanese Patent Laid Open Nos. 200215/85, 200216/85, 71915/87 and 174711/87.
The above conventional projection lenses have a short projection distance and have a good focusing characteristic, but have not always been satisfactory with respect to contrast on the projection screen. In view of this point, that is, for improving the contrast, the Japanese Patent Laid Open No. 174711/87 (corresponding to U.S. Pat. No. 4,884,879) referred to above proposes a technique whereby the space between an outer surface of a cathode-ray tube panel having a fluorescent inner surface and a concave lens positioned on a screen side away from the outer surface of the CRT panel is filled with a medium having a refractive index close to that of the concave lens or the panel glass to diminish the reflection on the panel-side interface of the concave lens, thereby improving the contrast. In this conventional technique, however, no consideration is given to the light reflected on the screen-side interface of the concave lens in contact with air, and the reflected light from the interface to the panel side is not removed thoroughly.
This problem will now be explained in more detail with reference to FIGS. 8 and 9 each illustrating a positional relation among a cathode-ray tube, the above concave lens as a part of a projection lens system and a medium. In FIGS. 8A and 9A, the numeral 10 denotes a cathode-ray tube; numeral 6 denotes a panel of the CRT 10; numeral 14 denotes a phosphor applied to the inner surface of the panel; numeral 15 denotes a metal back film formed of aluminum by vapor deposition on the inside of the phosphor; numeral 4 denotes a concave lens constituting the projection lens system and positioned nearest to the cathode-ray tube; and numeral 8 denotes a bracket for the connection of the concave lens 4 and the cathode-ray tube 10 and also for the sealing of a medium (functioning also as coolant) between the concave lens and the panel. The portion comprising the phosphor layer formed on the inner surface of the panel and the metal back film is normally called a fluorescent face P.sub.1. FIGS. 8B and 9B are each a view in which a ghost image induced by the reflected light on the air-side interface of the concave lens 4 is seen on the screen when a luminous point is formed on the fluorescent face of the panel 6.
FIG. 8B shows a state in which a luminous point A is positioned at the center of the fluorescent face P.sub.1. As shown in FIG. 8A, light rays .alpha. travelling gradually toward the center of the lens pupil (not shown) are partially reflected by the air-side interface of the concave lens 4 and return to the panel 6. This reflected light passes through the phosphor 1 and is again reflected by the metal back film 15, then appears on the screen as if the phosphor 14 were luminous as represented by portion B in FIG. 8B.
Further, light rays .beta., as shown in FIG. 8A, travelling still more outside the light rays .alpha. are partially reflected by the air-side interface of the concave lens 4 and return to the panel 6 like the light rays .alpha., but do not return to the phosphor 14 due to total reflection on the fluorescent face P.sub.1. Consequently, there appears a ring-like black band represented by C in FIG. 8B.
Additionally, light rays .gamma., as shown in FIG. 8A, travelling further outside the light rays .beta. return to the panel 6 after total reflection on the air-side interface of the concave lens 4. This reflected light, like the reflected light of the light rays .beta., is totally reflected by the fluorescent face P.sub.1 of the panel 6, but since the intensity thereof is very strong as compared with the reflected light of the light rays .beta., it is impossible to ignore a component of irregular reflection on concave and convex portions of the fluorescent face P.sub.1 (the fluorescent face P.sub.1 is an uneven or rough surface due to the application of the phosphor thereto), which component appears as a ghost image on the screen, as represented by D in FIG. 8B.
Now, FIGS. 9A and 9B will be described with respect to a case where a luminous point A is in an off-center position on the fluorescent face P.sub.1. A basic phenomenon is the same as that in the case where a luminous point is centrally positioned, but in the case of total reflection on the air-side interface of the concave lens 4, light rays .beta. reflected near the center of the concave lens are incident on the fluorescent face P.sub.1, of the panel 6 at a small angle .theta., so total reflection thereof does not occur on the fluorescent face P.sub.1. Consequently, as represented by E in FIG. 9B, a very bright, crescent-shaped ghost image is developed in an overlapped position of the ghost images B and D. The higher the contrast performance of a television set, the more conspicuous this ghost image becomes, resulting in degradation of the picture plane.
The reflection on the air-side interface of the concave lens is about 4%, which reflection can be reduced by forming a multi-layered film anti-reflection coating on the air-side of the concave lens. However, in the case where the concave lens is formed of an optical resin, it is impossible to form such multilayered film anti-reflection coating. More particularly, the concave lens faces a high-temperature coolant and thus the environment thereof is very restricted. The temperature range is from -20.degree. to +80.degree. C. On the other hand, the expansion coefficient of the optical resin is fairly large as compared with that of the coating material. Due to such a great difference in expansion coefficient, there occurs a breakage of the film as the temperature changes. Particularly, when the number of coating layers is large and the entire thickness of the multi-layer film is large, the contraction and expansion properties of the film are deteriorated and film breakage is apt to occur. Therefore, a coating of only coating of a single-layer film and properly be effected and in this case the reflection is 1 to 2%. Accordingly, a multi-layered anti-reflection coating cannot be formed on the concave lens and so far it has been impossible to fully eliminate the reflected light returning to the fluorescent face from the air-side interface of the concave lens.
The above-described construction intends to eliminate the source of the reflected light itself. On the other hand, there has also been proposed a construction wherein an optical element which absorbs the reflected light is provided between the fluorescent face and the air-side interface of the concave lens. For example, in Japanese Patent Laid Open No. 254890/85 there is disclosed a technique wherein a having a light absorbing wall is provided in a lower cathode-ray tube and the reflected light from the air-side interface of the concave lens is absorbed by the light absorbing wall to improve the contrast. However, such construction presents a problem with respect to brightness because an oblique exit light from the cathode-ray tube is wholly absorbed by the light absorbing wall. Additionally, the number of parts is increased because it is necessary to provide a louver as an optical element having a light absorbing wall and this also presents a problem.